Although the case sounds unique, it is becoming increasingly common in sting operations for undercover agents to introduce or suggest a gun as a form of payment in a drug deal. Under the law as written by Congress, the gun adds an automatic five years in prison – and sometimes much more – to any drug charges.

"It is a very heavy stick," says Mark Stancil, a Washington, D.C., lawyer and adviser to the University of Virginia School of Law Supreme Court Litigation Clinic, which is assisting in the Watson case.

The 2004 transaction

In November 2004, Michael Watson of Ascension, La., told an acquaintance he wanted to buy a handgun for protection.

The man, who is in his 50s and is legally blind, said he needed the gun to safeguard his possessions and his home after a couple of break-ins. But because Mr. Watson was a convicted felon, he was barred from purchasing a firearm from a licensed gun shop.

Unknown to Watson, his acquaintance was working as an informant for law-enforcement officials. Instead of a cash transaction, the officials told the acquaintance to suggest a drugs-for-gun barter deal between Watson and an undercover federal agent posing as an illegal gun dealer. Watson agreed to the swap.

Shortly after Watson exchanged the pills for the gun, he was placed under arrest. Federal authorities charged him with drug dealing and possession of a gun by a felon. But prosecutors didn't stop there. They also charged Watson with "using" a gun during a drug-trafficking offense – a crime that carries a five-year minimum mandatory sentence.

Prosecutors reasoned that since the unloaded pistol was "used" as payment for the drugs, the gun was used in a way that triggered additional criminal liability. Watson pleaded guilty to drug dealing and unlawful possession of the gun, but he is appealing the charge that he "used" the gun in the drug deal.

The unloaded gun provided by the undercover agent was under Watson's control for only a few moments before his arrest.

The justices must decide whether Watson's brief possession of the gun qualifies as a prohibited "use" in a drug transaction under the federal statute.

The Supreme Court confronted a similar issue in a 1993 case in Florida involving an attempt by a defendant to trade an automatic MAC-10 machine gun for cocaine. The court ruled 6 to 3 that the transaction amounted to "use" under the law.

"The fact that a gun is treated momentarily as an item of commerce does not render it inert or deprive it of destructive capacity," Justice Sandra Day O'Connor wrote for the majority.

But Watson's case is different, says Mr. Stancil. The 1993 Florida case involved a defendant trading his own gun for drugs. Watson's case involves the inverse – a defendant trading his own drugs for a gun.

Stancil says the juxtaposition makes a huge difference in how the high court may interpret the word "use" in the statute.

He offers an example. "It is natural to say I 'used' $1 to buy a cup of coffee," he says. "But it is completely unnatural to say I bought a cup of coffee with $1 and thereby 'used' the coffee."

The lawyer adds, "In the plain and ordinary meaning of the statute, it makes no grammatical sense to say that the buyer has used the object he is trying to purchase simply by completing the transaction."

"By receiving a gun as payment for drugs, Watson did not 'use' that firearm," Stancil writes in his brief to the court. "Because Watson did not 'use' the firearm in any ordinary recognizable sense of the word – let alone 'actively employ' it – his conviction must be reversed."

Firearms' potential harm

Government lawyers disagree.

It was Watson who asked for a gun, says Solicitor General Paul Clement in his brief. "By contributing to the introduction of the firearm into the transaction, [Watson] caused the very harm that Congress sought to avoid in enacting [the gun law]," he says.

"Regardless of which side of the guns-for-drugs barter a defendant is on, the firearm's presence – and its integral role in the drug deal – causes the risk to society that Congress sought to prevent," Mr. Clement says.

Although the Supreme Court resolved the "gun for drugs" issue in 1993 and reaffirmed it in a subsequent decision in 1995, the high court has never directly address the "drugs for gun" issue.

The federal appeals courts are split. Six side with the government's view. Four embrace the approach outlined by Stancil.

When the issue arose at the high court in 1993, the question that divided the justices was whether Congress meant to outlaw the use of guns as weapons.

Justice O'Connor and five other justices rejected that narrow reading of the law. "We … see no reason why Congress would have intended courts and juries applying [the law] to draw a fine metaphysical distinction between a gun's role in a drug offense as a weapon and its role as an item of barter," she said. "It creates a grave possibility of violence and death in either capacity."

Three justices dissented. Justices Antonin Scalia, John Paul Stevens, and David Souter said under the ordinary meaning of the word "use," the statute outlaws the use of a gun as a weapon – not for barter.